1. Field of the Invention
The invention relates generally to a method or system for interfacing balanced and unbalanced stages in an electronic system and more particularly to interfacing a balanced input stage, providing output signals which are 180.degree. out of phase, and an unbalanced load which driven by a single-ended input signal.
2. Description of the Relevant Art
A power amplifier is an amplifier having an output which is considered in terms of its power capability instead of its voltage or current limitations. Relatively high output power is required when the amplifier is used to operate a motor, drive a loudspeaker or an antenna, or perform some other similar task.
The power developed by the amplifier must be efficiently transferred to a load.
In many instances the power required by the load is more than can be generated by single amplification element such as a transistor. One effective way to connect more than one amplification element to increase power is the push-pull configuration depicted in FIG. 1.
In FIG. 1, a balanced, push-pull amplifier includes two amplification elements 12a and b which are, for example, NPN transistors having their sources coupled to ground and their gates coupled to receive, respectively, first and second input signals which are 180.degree. out-of-phase (odd-mode excitation). In low-frequency push-pull amplifiers, a transformer 14 is typically utilized to couple the balanced, push-pull amplifier 10 to an unbalanced load 16. Thus, the transformer performs the function of a balun, i.e, coupling a balanced, or differential, signal source to an unbalanced, or single-ended, load.
Various input-stages for providing the out-of-phase input signals to the push-pull amplifier, e.g., an active splitter with a differential pair, are well-known in the art. A transformer can also provide this unbalanced to balanced input function. In practice, these input stages may also provide a common mode signal that has the same phase (in-phase or even-mode excitation) at both inputs of the push-pull amplifier. For example, a biasing voltage is often applied as a common mode voltage signal.
Circuit designers need a balun that rejects common mode signals and provide equal balanced and unbalanced port currents and voltage levels. It is also desirable to isolate D.C. voltage levels from the following stage.
The balun functions are typically achieved by utilizing a magnetic coupling. A solution utilizing a magnetically coupled tri-filar balun is depicted in FIGS. 2A and B, where an auto-transformer implementation is depicted.
FIGS. 3A and 3B depict, respectively, how the magnetic coupling causes the rejection of even mode excitations and the transmission of balanced excitations while providing equal balanced and unbalanced port currents.
Although magnetically coupled structures function well as baluns it is very difficult to integrate such structures into integrated semiconductor circuits. Thus, the reduction in size and other beneficial properties of MMICs (monolithic microwave integrated circuits) are not readily realized by baluns utilizing magnetic coupling.